CHAPTER 28 The Cephalic, Oral, and Esophageal Phases of the Integrated Response to a Meal the responses initiated by the presence of food in the oral cavity are identical to those initiated in the cephalic phase, because the efferent pathway is the same. The responses specifically initiated in the mouth, which consist mainly of the stimulation of salivary secretion, will be discussed next.
The mouth is important for the mechanical disruption of food and for initiation of digestion. Chewing subdivides and mixes the food with the enzymes salivary amylase and lingual lipase and with the glycoprotein mucin, which lubricates food for chewing and swallowing. Minimal absorption occurs in the mouth, although alcohol and some drugs are absorbed from the oral cavity, and this can be clinically important. However, as with the cephalic phase, it is important to realize that stimulation of the oral cavity initiates responses in the more distal GI tract, including increased gastric acid secretion, increased pancreatic enzyme secretion, gallbladder contraction, and relaxation of the sphincter of Oddi, mediated via the efferent vagal pathway.
Properties of Secretion
Secretions in the GI tract come from glands associated with the tract (salivary glands, pancreas, and liver), from glands formed by the gut wall itself (e.g., submucosal glands in esophagus and duodenum), and from the intestinal mucosa itself. The exact nature of the secretory products can vary tremendously, depending on the function of that region of the GI tract. However, these secretions have several characteristics in common. Secretions from the GI tract and associated glands include water, electrolytes, protein, and humoral agents. Water is essential for generating an aqueous environment for efficient enzyme action. Secretion of electrolytes is important for generation of osmotic gradients to drive the movement of water. Digestive enzymes in secreted fluid catalyze the breakdown of macronutrients in ingested food. Moreover, many additional proteins secreted along the GI tract have specialized functions, some of which are fairly well understood, such as those of mucin and immunoglobulins, and others that are only just beginning to be understood, such as those of trefoil peptides.
Secretion is initiated by multiple signals associated with the meal, including chemical, osmotic, and mechanical components. Secretion is elicited by the action of specific effector substances called secretagogues acting on secretory cells. Secretagogues work in one of the three ways that have already been described in
Chapter 27—endocrine, paracrine, and neurocrine.
Constituents of Secretions
Inorganic secretory components are region or gland specific, depending on the particular conditions required in that part of the GI tract. The inorganic components are electrolytes, including H+ and HCO3 − . Two examples of different secretions include acid (HCl) in the stomach, which is important to activate pepsin and start protein digestion, and HCO3 − in the duodenum, which neutralizes gastric acid and provides optimal conditions for the action of digestive enzymes in the small intestine.
Organic secretory components are also gland or organ specific and depend on the function of that region of the gut. The organic constituents are enzymes (for digestion), mucin (for lubrication and mucosal protection), and other factors such as growth factors, immunoglobulins, bile acids, and absorptive factors.
During the cephalic and oral phases of the meal, considerable stimulation of salivary secretion takes place. Saliva has a variety of functions, including those important for the integrative responses to a meal and for other physiological processes (
Box 28.1 ). The main functions of saliva in digestion include lubrication and moistening of food for swallowing, solubilization of material for taste, initiation of carbohydrate digestion, and clearance and neutralization of refluxed gastric secretions in the esophagus. Saliva also has antibacterial actions that are important for overall health of the oral cavity and teeth.